In known activated sludge treating apparatuses of this type, a plurality of filter membrane modules are vertically arranged in parallel spaced relation within a treating tank, whereby the water to be treated is separated into solid and liquid by being passed through the filter membrane modules so that water that has passed through the filter membrane modules is extracted as treated water. In such known type of apparatus for treatment of activated sludge, a diffused aeration pipe having a plurality of jet holes is disposed at a level below the filter membrane modules in order to prevent deposition and growth of cake and gel layers on the outer surfaces of the filter membrane modules. The air lift action of air blow from the diffused aeration pipe serves to create upward flowing agitation currents of both gas and liquid phases which are utilized to clean the membrane surfaces of the filter membrane modules.
According to such arrangement, it is possible to enhance the cleaning effect of such agitation currents on the filter membrane modules to a certain extent by increasing the amount of air blow from the diffused aeration pipe. However, this involves the following problems. Increased air blow from the diffused aeration pipe may result in excessive aeration with respect to the water being treated and, especially where the water being treated contains ammonia, the pH value of the water will be excessively lowered, the operation of activated sludge treating being thus unfavorably affected.
Another problem is that when the rate of air blow exceeds a specific value, for example, about 1 (m.sup.3 /m.sup.3 .multidot.min., where denominator m.sup.3 represents cross-sectional area of flow path of gas-liquid phase currents), the air lift action of air blow is decreased due to air-bubble coalescence, which results in decreased energy efficiency. A further problem is that the distance between the diffused aeration pipe and the lower end of the filter membrane modules is so short that air blown from the diffused aeration pipe is unlikely to be uniformly dispersed, which fact will not only tend to decrease the air lift effect of air blow but may tend to create some dead space in filter membrane modules which will remain isolated from gas-liquid phase current flow. As a consequence, cake layers or the like will deposit and grow on such portion of filter membrane modules, thus lowering the flux of the filter membrane modules.